Fiber optic communication links have been conventionally employed in long-haul, point-to-point networks with controlled environments at all interface points. Such highly controlled, “central office” surroundings usually offer relatively benign operating environments (temperature, humidity, mechanical) for components. Consequently, highly functional components could be developed and installed without considering the impact of other, more extreme environments.
Recent technological advances, coupled with increasing bandwidth demand, are rapidly expanding the use of fiber optic components beyond the “central office” and into potentially harsher environments. For example, dense wavelength division multiplexing (DWDM) enables the transmission of multiple, independent wavelength streams across a single fiber. Predictably, this capability has resulted in the requirement to add or drop these optical channels along the previously untapped long lengths of fiber (and outside of the central office environment) to provide access to the individual wavelength streams. Optical add/drop multiplexers (OADMs) are employed for this function, enabled by arrayed waveguide grating (AWG) components for filtering and forwarding individual wavelengths.
In addition to these technological advances, simple market forces are pushing fiber networks beyond central offices and into the diverse terrain of “metro” markets. This ever-increasing need for bandwidth which only fiber can deliver is resulting in the widespread deployment of fiber networks, and their associated components, into the harsher, less environmentally controlled conditions present in the metro market.
The demands placed on component designers now reach far beyond optical performance, and into the realms of thermal, humidity and mechanical insulation. Certain qualification standards (e.g., Telcordia) exist for reliability of optical components, and many customers require qualification under these standards. AWGs however are thin, fragile chips with narrow waveguides produced using planar lightwave circuit (PLC) processing techniques. The various processing tolerances required to meet the requisite optical specifications are already very tight, and in fact get tighter as the need to process more and closer channels increases.
One particular concern for PLC waveguides, including those in AWGs, is their sensitivity to stress imbalances, and the impact of stress imbalances on optical performance. These stresses can be induced by the environmental conditions discussed above, and by the fabrication process itself. Stress-induced birefringence in waveguides leads to unacceptably high polarization dependent loss (PDL) for communication systems.
Waveguides are typically fabricated by forming (e.g., etching) waveguide core patterns over a substrate and undercladding. A doped glass overcladding (e.g., boro-phosphate silicate glass or BPSG) is then formed over the cores, to complete the waveguide formation. Because the materials used for these layers are different, with differing properties (e.g., differing coefficients of thermal expansion (CTEs)), intra-and inter-layer stresses exist and will result in high levels of waveguide PDL.
Techniques have been disclosed to address these problems, such as stress release grooves (SRGs) (see, e.g., Nadler et al, “Polarization Insensitive, Low-Loss, Low-Crosstalk Wavelength Multiplexer Modules,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, No. 5, September/October 1999) and tailoring of the overcladding (see, e.g., Kilian et al, “Birefringence Free Planar Optical Waveguide Made by Flame Hydrolysis Deposition (FHD) Through Tailoring of the Overcladding,” IEEE Journal of Lightwave Technology, Vol. 18, No. 2, February 2000; and “Simple Method of Fabricating Polarisation-Insensitive and Very Low Crosstalk AWG Grating Devices,” Electronics Letters, Vol. 34, No. 1, Jan. 8, 1998). Such techniques are broadly referred to herein as stress management or stress engineering, which in effect “balance” the stress affecting the waveguides. The term “balance” is known to those in the art and used broadly herein to connote any type of active stress management which provides the requisite, advantageous minimization of birefringence. Multiple stress balancing techniques are disclosed herein.
Even assuming that such techniques are employed to manage stress, they are still susceptible to the adverse environmental conditions, discussed above. However, any techniques used to protect the circuit from these environmental conditions must also be compatible, and not interfere with, any stress management techniques employed. Modified annealing techniques for the overcladding have been proposed, but have not produced satisfactory protection. Hermetic packaging of the circuits can also provide protection, but such techniques can be expensive, and subject to long-term failures. To decrease reliance on packaging, what is required are advanced techniques to protect planar lightwave circuits from adverse environmental conditions, while maintaining their stress engineered properties at the chip level.